A polyvinyl acetal represented by polyvinyl butyral has excellent adhesion and compatibility with various organic and inorganic substrates, and has excellent solubility in an organic solvent. The polyvinyl acetal is widely used as various adhesives, a binder for ceramics, various inks, paints, and the like, or an interlayer film for safety glass.
Recently, in an application of an interlayer film for laminated glass, various highly functionalized products have been developed. For example, a laminated sound insulation interlayer film for laminated glass is disclosed (for example, refer to Patent Documents 1 and 2). In the laminated sound insulation interlayer film for laminated glass, in order to impart high sound insulating properties to the interlayer film for laminated glass, a plurality of polyvinyl acetal layers having different compositions is laminated. For example, the polyvinyl acetal layers have different content ratios of a polyvinyl acetal and a plasticizer. In the interlayer film for laminated glass, in general, in order to make amounts of a plasticizer included in the respective layers different from each other, polyvinyl acetals having different average amounts of remaining hydroxyl groups between the respective layers are used.
By the way, in general, an interlayer film for laminated glass is manufactured using an extruder from a viewpoint of production costs thereof. The laminated sound insulation multilayer interlayer film for laminated glass is manufactured by a coextrusion method. However, when the interlayer film for laminated glass is manufactured by this method, a trim or an off-spec product which cannot be a commercial product because of nonuniform composition or thickness is obtained.
In general, such a trim or an off-spec product can be recycled by melt kneading and extrusion molding. However, when a mixture of polyvinyl acetal compositions having different compositions as a trim or an off-spec product is used, a resulting interlayer film for laminated glass may be opaque. In particular, in the laminated sound insulation interlayer film for laminated glass, the polyvinyl acetal compositions included in the respective layers include polyvinyl acetals having different average amounts of remaining hydroxyl groups from each other. Therefore, it is difficult to make the polyvinyl acetals compatible with each other. The interlayer film for laminated glass obtained by recycling the trim or the off-spec product generated during manufacturing the interlayer film for laminated glass has a problem of poor transparency.
When a polyvinyl acetal resin is manufactured, a batch type precipitation method has been industrially and widely performed.
In a typical batch type precipitation method, polyvinyl alcohol as a raw material of the polyvinyl acetal resin is dissolved in water. An aldehyde such as butylaldehyde, and an acid catalyst such as hydrochloric acid or nitric acid are added thereto to initiate an acetalization reaction. When an acetalization degree reaches a certain level, partially acetalized polyvinyl alcohol which cannot be dissolved in water precipitates to become particles. Thereafter, the acetalization reaction proceeds in a heterogeneous system in which the particles are dispersed in water. After completion of the reaction, the acid catalyst is appropriately neutralized, washed, and dried to obtain a polyvinyl acetal.
A resin obtained by the batch type precipitation method is preferable because the resin is obtained in a form of particles, and excellent in purifying the resulting resin by washing with water and in handling the resin after drying. Meanwhile, because of the batch reaction, even when production conditions are the same, average acetalization degrees of the polyvinyl acetals obtained for batches may vary between the batches (that is, an average amount of remaining hydroxyl groups (mol %) may be different). Because of such a variation between the batches, when two or more kinds of polyvinyl acetals having average acetalization degrees different from each other by a certain degree or more are mixed with a plasticizer generally used for a polyvinyl acetal, such as triethylene glycol di-2-ethylhexanoate (3GO), to be used, transparency of a resulting composition may be lowered.
As described above, the acetalization reaction after particle precipitation proceeds in a heterogeneous system. Therefore, in some reaction methods, a polyvinyl acetal (a polyvinyl acetal having a large variation in a batch) having a relatively wide distribution of the acetalization degree even in the same batch (that is, variation of an amount of the remaining hydroxyl groups (mol %) in a batch is large) may be obtained. Particularly, when the acetalization reaction is performed in such a reaction condition as to increase a particle diameter of a polyvinyl acetal, there is a difference in an aldehyde concentration and an acid catalyst concentration between the inside and the outside of the particles during the reaction. Therefore, a polyvinyl acetal having a relatively wide distribution of the acetalization degree even in the same batch tends to be obtained. When the polyvinyl acetal having a relatively wide distribution of the acetalization degree even in the same batch is mixed with 3GO, transparency of a resulting composition may be lowered. Particularly, laminated glass including, as an interlayer film, a sheet made from a composition may be clouded under irradiation with light, the composition being obtained by mixing the polyvinyl acetal having a relatively wide distribution of the acetalization degree even in the same batch and a plasticizer generally used for a polyvinyl acetal, such as 3GO. An application thereof in exposure to strong light, for example, an application thereof to a windshield of a car (exposed to a headlight from an oncoming car during driving at night) is difficult.